Method of and apparatus for detecting gap conditions in EDM process with monitoring pulses

ABSTRACT

Method of and apparatus for controlling electric discharge machining processes in which machining pulses are applied across a machining gap with a pulse duration and peak current preselected to attain a predetermined machining consequence or result. At least one monitoring pulse is interposed in the succession of machining pulses and is time-spaced from and totally independent of the machining pulses while being dimensioned to give rise to a current pulse in the machining gap. The current pulse is measured as to at least one characteristic and the gap condition is determined in response to this measurement. sp 
     CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending application Ser. No. 860,164 filed Dec. 13, 1977 (U.S. Pat. No. 4,236,057).

FIELD OF THE INVENTION

The present invention relates generally to electrical dischargemachining, commonly called EDM, in which time-spaced discrete electricaldischarges are effected across a machining gap formed between a toolelectrode and a workpiece spacedly juxtaposed therewith to removematerial from the workpiece. The invention is particularly concernedwith a method of detecting gap conditions in EDM processes and anapparatus for carrying out the method.

BACKGROUND OF THE INVENTION

One of major problems in the art of EDM is detecting electrical orphysical conditions in the machining gap. Thus, according to particulargap conditions existent or encountered in the EDM process, it isnecessary, for example, to modify the application of discharge-producingelectrical pulses to the machining gap, to control the rate of flushingthe gap with the machining fluid and to regulate the conductivity of themachining medium. Further, once the machining gap suffers acontamination with machining chips, tar and other products which tend tobring about arcing, it is imperative that the gap-cleaning action beeffectuated or intensified by means of vibration or retraction of thetool electrode relative to the workpiece. The follow-up feed orservo-displacement of the electrode must also be smoothly effected asmachining proceeds and yet in response to change in the gap conditions.

There have in the past been proposed a number of gap-detecting methods,which may be classified into two groups. The first is to detect the gapvoltage, current and/or other gap variable on an average basis and thesecond is to sense such variables on a per-pulse basis. The averagingmeasurement is obviously less reliable because of its inability ofinstantaneous response and is therefore not adequate for promptcorrective action. On the other hand, the per-pulse measurement maypermit immediate countermeasures and can accordingly afford an enhancedmachining efficiency. The problem is, however, that machining conditionsthemselves may affect parameters of individual machining pulses (i.e.peak current and on time, etc) which must be set at optimum valuesaccording to particular machining purposes (i.e., for obtaining adesired relationship of surface roughness, overcut, relative electrodewear, etc.). Hence the attempt to judge the machining conditions bysensing variables of machining pulses themselves leads most often tofalse results and a truly accurate determination of the gap conditionsis not obtainable without adequately incorporating a change inparameters of machining pulses into sensing signals.

In a further attempt to detect gap conditions, there has also beenintroduced in the art pilot pulses or discharges which are usedauxiliary to machining discharges. In the pilot-pulse methods which havebeen contemplated heretofore, a pilot signal is applied at a frontal orleading portion of each individual machining pulse to explore apre-discharge gap condition for determining whether the gap would be inan adequate state so that the machining pulse may be triggered or forother control purposes. In these methods, the "pilot pulse" is more orless integrated with the subsequent "machining" portion of eachindividual power pulse so that there may also be an adverse influencetherefrom on each machining pulse with a set of prefixed parameters.Consequently the disadvantages mentioned earlier remain unresolved.

OBJECT OF THE INVENTION

It is therefore the object of the present invention to provide an EDMmethod and apparatus whereby gap conditions can be detected withoutinterference with machining so that a judgement of machining conditionsis obtainable with increased accuracy and reliability with the resultwhich is highly useful for controlling various machining parameters inthe manner to achieve a desired machining consequence (i.e. relationshipof surface roughness, overcut, relative electrode wear, etc.) at anoptimum efficiency.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method ofdetecting EDM gap conditions, comprising the steps of applying across amachining gap a succession of machining pulses having a pulse durationand peak current preselected to attain a desired machining consequence;interposing into said succession at least one monitoring pulse eachtime-spaced and independent from adjacent machining pulses, saidmonitoring pulse resulting in a gap current pulse having a pulseduration and peak current preselected independently from the machiningpulses; measuring characteristics of the resulting monitoring gapcurrent pulse thereby classifying it into one of at least twocategories; and ascertaining the gap condition in response to the resultof the preceding step, i.e. the classification.

In accordance with a specific aspect of the invention, the monitoringpulse has a pulse duration in the range between 5 and 20 microsecondsand a pulse peak current in the range between 10 and 100 amperes. Thepulse duration and peak current of the monitoring pulse, in combination,are, however, selected within said respective ranges preferably so as tobe capable of achieving essentially the same machining performance asthe machining pulse having different pulse duration and peak current.For example, since there is a relationship empirically establishedbetween surface roughness and pulse duration and peak current andexpressed as Ra=Kτ_(on) ⁰.3 ·I_(p) ⁰.4, if the machining pulses usedhave a pulse duration τ_(on) of 3 microseconds and a peak current I_(p)of 10 amperes, then the pulse duration of monitoring pulses may be 10microseconds with a peak current of 4 to 5 amperes. The monitoringpulses are applied, for example, at a frequency between 1 and 1000 Hz inconjunction with machining pulses applied at a frequency between 100 Hzand 500 kHz.

The measurement of a characteristic of the gap current signal resultingfrom the application of a monitoring pulse may be effected by measuringthe voltage magnitude or another variable of the gap signal forcomparison with a threshold value or a plurality of reference valuespredetermined for classification of gap discharges. The measurement mayalso be effected over each monitoring pulse or for a predetermined timeperiod thereof excluding the instance of spark-over or gap breakdownwhich is transient in nature. It is also advisable to choose for thesensing time the end of each monitoring discharge by having the sensingtime the end of each monitoring discharge by having the sensing systemresponsive to the cut-off signal of each monitoring pulse.

Gap discharges resulting from individual monitoring pulses may beclassified into "good" and "bad" by ascertaining where the measuredvariable lies with reference to a threshold level. The "good" dischargesmay further be classified into "metallic phase" and "gas phase"discharges. The "gas phase" discharge has a discharge voltage of 15 to25 volts and a high-frequency oscillating component of 5 to 20 volts andis produced purely by the discharge through a gas phase. The "metallicphase" discharge has a voltage of 10 to 15 volts and a high-frequencycomponent of 2 to 5 volts and is produced at the end of discharge orupon clarification of an interelectrode gap short-circuiting by amachining chip. The characteristics of these types of discharges and acombination thereof will be more fully described hereinafter.

The determination of the gap conditions is effected advantageously byapplying discharge-classifying signals through the respective channelsto preset counters or a reversible counter to obtain collectivelydiscriminated results so that development and tendency of machiningconditions may be assured to provide control signals for one or more ofthe controllable systems mentioned earlier.

In accordance with a further aspect of the invention, the classificationof discharges resulting from monitoring pulses of a preselected pulseduration and peak current is effected advantageously by sensing themagnitude of high-frequency oscillatory current or voltage contained inthe discharges mentioned already. As is well known in the art, if thedischarge is of arcing or short-circuiting type, the high-frequencycomponent is absent or not observable therein. It has thus only beenrecognized that a discharge is "normal" if such a component exists. Theinvention in this aspect is based upon the discovery that thehigh-frequency oscillatory discharge condition is affected by variousmachining factors including machining fluid and a combination ofelectrodes and is determinative of characteristics of resultingmachining discharges (e.g., metallic-phase discharges and gas-phasedischarges) and that a clear demarcation to this end is obtainable bythe introduction of monitoring pulses of prefixed parameters independentof the machining pulses. The high-frequency component, which isapparently caused by electron avalanches of a duration of 0.03 to 1microsecond has a frequency in the range between 1 and 30 MHz and amagnitude which, together with the discharge voltage magnitude, variesas a function of the molecular weight of the machining fluid used. Thehigh-frequency oscillation tends to damp with time within each dischargeand is distinctively observable for a period up to 15 microseconds andwith a peak current not exceeding 100 amperes, in the range of eachparameter given each individual monitoring pulse according to theinvention. It is convenient to sense the high-frequency component in thegap upon, as referred to earlier, termination of a switching signalwhich applies each monitoring pulse since this avoids a transient periodat the beginning of the discharge and because of a good timingexpediency.

The high-frequency oscillatory voltage may be sensed also as thedifference of its magnitude at the commencement of each monitoringdischarge minus its magnitude at a given instant within the dischargedivided by the time duration of this period.

BRIEF DESCRIPTION OF THE DRAWING

Certain embodiments of the invention will be described in the followingwith reference to the accompanying drawing in which:

FIGS. 1A, B, C, D and E are schematic waveform diagrams showing variousmodes of monitoring pulse interposition into successions of machiningpulses in accordance with the principles of the present invention;

FIG. 2 is a circuit diagram illustrating a gap-condition detector systemfor embodying the present invention;

FIG. 3 is a schematic waveform diagram illustrating machining pulses anda monitoring pulse produced in the system of FIG. 2;

FIG. 4 is a schematic waveform diagram illustrating a voltage waveformof a discharge containing a high-frequency oscillatory component;

FIG. 5 is a graphical representation showing the magnitudes of dischargevoltage and high-frequency component thereof which vary as a function ofthe molecular weight of machining fluid;

FIG. 6 is a circuit diagram illustrating a gap-condition detector systemwith a machining pulse and monitoring power supply arrangement of FIG. 2and including a sensing unit responsive to the high-frequencyoscillatory discharge component;

FIG. 7 is a circuit diagram illustrating a servo-control assemblyincorporating a gap-condition detector system according to theinvention;

FIG. 8 is a waveform diagram similar to that of FIG. 4 and with anemphasis on the damping of the high-frequency component;

FIG. 9 is a diagrammatic illustration of the high-frequency componentalone extracted from a discharge voltage waveform;

FIG. 10 is a circuit diagram of a further gap-condition detector systemresponsive to the high-frequency component;

FIGS. 11A, B and C are schematic waveform diagrams illustratingdifferent forms of discharge;

FIG. 12 is a graphical representation of stock removal versus differencein phase of discharge;

FIG. 13 is a graphical representation of mean gap voltage versusdifference in phase of discharge; and

FIG. 14 is a circuit diagram for discriminating between different formsof discharge according to the invention.

SPECIFIC DESCRIPTION

Monitoring pulses with preselected pulse duration (τ_(on)) and peakcurrent (I_(p)) according to the invention may be interposed intomachining pulses in any of various manners as illustrated in FIGS. 1A-Ewith the ratio of monitoring pulses to machining pulses less than 1.Shown at A are monitoring pulses of a rectangular form and with a peakcurrent lower than that of machining pulses as monitoring pulses of B.In the latter case, monitoring pulses have a pulse on-time or durationgreater than machining pulses. In the examples of C, monitoring pulsesare shown periodically increasing its number from one, two to three. InD and E, monitoring pulses have a triangular and a halfwave rectified ACwaveform, respectively.

As noted earlier, machining pulses are preset with a pulse duration(τ_(on)) and a peak current (I_(p)) which in combination are capable ofattaining desired machining results, e.g., surface roughness (R) inμRmax, overcut (δ_(out)) in mm and relative electrode wear (γ) in volume%. In this connection the following relationships have empirically beenestablished: R=Kr·τ_(on) ⁰.3 ·I_(p) ⁰.4, δ_(out) =Kδ_(O) ·τ_(on) ⁰.3·I_(p) ⁰.3, γ=K·τ_(on) ⁻⁰.1 ·I_(p) ⁻⁰.2 ("wear" mode) and γ'=Kr'·τ_(on)⁻¹.8 ·I_(p) ("no wear" mode). Monitoring pulses on the other hand arepreset with a pulse duration in the range between 5 and 20 microsecondsand a pulse peak current in the range between 10 and 200 amperes forconsideration of ease of discrimination between distinct characteristicdischarges as will be described in more detail hereinafter. Whilemonitoring pulses are thus preset independently from presetting ofmachining pulses, it is yet possible in consideration of theaforementioned relationships to have a monitoring pulse achievesubstantially the same machining performance as a machining pulse. Forexample, with machining pulses of a preset duration of 3 microsecondsand peak current of 10 amperes, monitoring pulses may be set with 10microseconds and 4 to 5 amperes so that a substantially identicalsurface roughness may result from both pulses. The number of monitoringpulses in typical EDM operations may be in the range between 1 to 10 persecond.

In FIG. 2 there is shown a system for applying, in accordance with thepresent invention, both machining pulses and monitoring pulses to an EDMgap G formed between a tool electrode 1 and a workpiece 2 to provide atits output a sensed information signal S resulting from the gapmonitoring. The machining gap G is typically filled and flushed with adielectric liquid which forms a machining medium and also serves tocarry away machining products from the region of the gap G.

The system of FIG. 2 includes a DC source 3 forming a common powersupply for machining and monitoring pulses and connected in series withthe machining gap G via parallel-connected branches respectivelyincluding a pair of switching elements 4 and 5 shown as transistors. Theswitch 4 is controlled by a pulser 6 via an AND gate 7 and therebyturned on and off to alternately connect and disconnect the DC source 3to the electrode 1 and the workpiece 2 at a frequency determined by thepulser 6. As a consequence, a succession of machining pulses areproduced across the machining gap G with a pulse duration (τ_(on))established by setting at the pulser 6. The switch 4 has an adjustableresistor 8 connected in series therewith for establishing the peakcurrent (I_(p)) of the machining pulses.

The pulser 6 has an additional output fed to a counter 9 which isadapted to count a preset number of output pulses from the pulser 6 forresetting thereupon while issuing a timing signal. The timing signal isapplied to a first timer 10 which may be constituted by a monostablemultivibrator that provides a pulse of a predetermined first durationand is adapted to trigger, upon termination of said duration, a secondtimer 11 which may again be a monostable multivibrator that provides apulse of a predetermined second duration. This latter pulse having apulse duration (τ_(on) ') is applied from the first output 11a of thetimer 11 to turn it on thereby connecting the DC source 3 to theelectrode 1 and the workpiece 2. The same pulse or a modified signaltherefrom is fed at the second terminal 11b of the timer 11 to one inputterminal of the AND gate 7 via an inverter 12 to disable the AND gate 7which is normally enabled. Consequently, the machining pulse signal isinhibited from passage to the switch 4 and a monitoring pulse isproduced across the machining gap G through the switch 5. The latter hasan adjustable resistor 13 connected in series therewith for establishingthe peak current (I_(p) ') of the monitoring pulse. The pulse durationof the monitoring pulse (τ_(on)) is, of course, determined and set atthe timer 11. When the monitoring pulse is terminated, the AND gate isagain enabled and a preselected number (set at the counter 9) ofmachining pulses recommence. This mode of the last machining pulse in atrain and the first machining pulse in the subsequent train interposedby a monitoring pulse is apparent from FIG. 3.

The timers 10 and 11 may also be adapted to provide an inhibiting signalfor disabling a number of machining pulses to be passed through the gate7 and to provide several monitoring signal pulses to be applied to theswitch 5 for effecting the several monitoring pulses during the timeperiod in which the machining pulses are inhibited. Further, dependingon the size of the pulse off-time of machining pulses relative to theiron-time, of course, one or more monitoring pulses may be produced duringthe normal pulse off-time of machining pulses with a proper setting oftimers 10 and 11.

A sensing resistor or potentiometer 14 connected across the machininggap G between the electrode 1 and the workpiece 2 feeds into thresholdcircuit 15 having another input fed by a third output terminal 11c ofthe timer 11 so that the gap voltage during the application of eachmonitoring pulse detected at the sensing resistor 14 is discriminatedwith reference to one or more threshold levels set in the circuit 15 todetermine the type of discharge resulting from the application of eachmonitoring pulse.

For the purpose of classification of discharges, while the gap voltagemay satisfactorily be used, a more accurate determination is obtained bymeasuring high-frequency oscillatory components contained in thedischarge voltage and current. In FIG. 4 there is shown the waveform of"normal" discharge voltage resulting from the application of amonitoring pulse with a preselected pulse duration and peak currentadequate for monitoring. The high-frequency oscillatory component ΔV ofa frequency or frequencies in a range between 1 and 30 MHz is containedin the discharge and superimposed on its DC component Vg. Once themachining gap is short-circuited or arcing develops, this oscillatorycondition disappears or becomes substantially unobservable. Thus,measuring the presence or absence of this component provides adequateinformation for gap monitoring.

It has now been discovered further that the magnitude of thehigh-frequency oscillatory component has also a dependency on the kindof machining medium. Thus, for example, with pure water (havingmolecular weight of 18) utilized as machining fluid, the magnitude ΔV isextremely small. The high-frequency magnitude ΔV is increased withkerosene (having molecular weight of 200 to 300) and further withspindle oil (having molecular weight of about 700). FIG. 5 is a graphrepresenting experimental results in which the gap voltage Vg is plottedalong the ordinate and the molecular weight of machining fluid isplotted along the abscissa. The graph shows that the gap voltage Vgwhich is the sum of anodic potential drop Va, cathodic potential drop Vcand high-frequency voltage component ΔV increases as the latterincreases with Va+Vb being constant, with the increase in the molecularweight of machining medium. It has been observed that ΔV amounts toapproximately 10 volts with spindle oil which is high in molecularweight. An assumption for the phenomenon is that electrons emitted fromthe cathode are more readily absorbed into molecules of higher molecularweight medium, causing an enhanced pulsation of discharge. With a mediumof lower molecular weight, electrons will less meet barriers so that amore continuous discharge with an increased number of free electrons mayresult. However, when a medium which is high in molecular weight isdegraded into lower molecular weight fragments by cracking duringmachining discharges, there will develop an arc or arc-like dischargewith less high-frequency oscillation. Accordingly, the classification ofdischarges with an increased accuracy is obtainable by judging thesensed high-frequency component signal with reference to a particularlevel corresponding to the particular molecular weight of machiningfluid employed. It should also be noted that the frequency of ahigh-frequency component is essentially proportional to its magnitudeand may be used instead of the latter for sensing.

It has already been mentioned that the pulse duration and peak currentof a monitoring pulse must be selected such as to allow an optimummonitoring. When the peak current employed is excessively high, thermalelectrons will increase excessively tending to cause larger molecules tobe decomposed into excessively smaller molecules so that thehigh-frequency component may disappear. The high-frequency oscillationalso diminishes with the lapse of discharge time as larger moleculesdecompose. Hence it is necessary that the monitoring pulse be shapedproperly both in width and magnitude, independently of machining pulseswhich must be shaped for consideration of machining consequences.

In FIG. 6 there is shown a device including a machining pulse andmonitoring pulse supply system 20 which is here essentially equivalentto that shown in FIG. 2 and a gap-condition detector system 21responsive to the high-frequency component in monitoring dischargepulses. In this system, the discharge voltage resulting from amonitoring pulse applied across the machining gap G is sensed by asensing network 22 which has a second input terminal energized by themonitoring pulser 11 and is hence responsive to the gap G selectivelyduring each monitoring pulse or a predetermined time period within orimmediately subsequent to each monitoring pulse. The sensed signal atthe network 22 is fed to a filter circuit 23 which extracts from theinput signal a high-frequency component (ΔV in FIG. 4) selectively thatis fed to an amplifier 24 which converts it into an amplified DC output.The next stage comprises a comparator 25 having a threshold value settherein for comparison of the incoming DC signal representative of themagnitude ΔV of the high-frequency component. The threshold value isvariably set by a preset network (reference-signal source) 26 inaccordance with the molecular weight of machining fluid utilized. Itwill be apparent that with water as machining fluid having a lowmolecular weight the threshold is set to be sensitive to a minimum valueand an increased level is used as the threshold for machining liquid ofa higher molecular weight as can be seen from FIG. 5. Should theincoming "high-frequency" signal be found to exceed the threshold levelindicating that the sensed discharge is a "good" discharge, a "1" signalmay be produced by a pulser 27 provided at the output of the comparator25 and may be forwarded to a counter unit 28. The latter may be a presetcounter adapted to count a present number of incoming pulses to providean output pulse upon such count indicating that machining continuesunder a satisfactory condition. Alternatively, the pulser 27 may issue a"1" signal in response to the comparator 25 determining the"high-frequency" signal to be less than the threshold and the presetcounter 28 may, upon counting a preset number of incoming "baddischarge" signal pulses, issue a signal indicating that machining isshifting into an unsatisfactory condition. It is also possible to havethe counter 28 constituted by an up/down counter having a pair of inputterminals for counting pulses from separate respective pulsers 27 (and27') coupled to the comparator 25 so that the counting level of theup/down counter indicates the degree of performance or satisfactorinessof machining in the gap. The output signal from the counter 28 may beutilized for the control of any one or a combination of machiningparameters, i.e. signals for electrode servomechanism, gap cleaning,modification of machining pulses etc.

In FIG. 7 there is shown a servo-control system for controlling therelative displacement between a tool electrode 1 and a workpiece 2 tomaintain the size of the machining gap G therebetween substantiallyconstant as machining proceeds. This system again includes a machiningpower pulse and monitoring pulse supply unit 20 mentioned previously.Also in this arrangement as well, a sensing resistor 14 is connectedacross the machining gap G to detect discharge voltage or current and,in conjunction with a threshold network 15, to provide signalscharacterizing monitoring pulse discharges effected. Thus, the thresholdnetwork 15 may be responsive directly to discharge voltage or, currentwith reference to a threshold level as in the embodiment of FIG. 2 or tothe high-frequency component thereof with reference to a threshold levelas in the embodiment of FIG. 6. Here again, the threshold network 15 mayactuate a pulser 30 to provide either one or both of digital signals,which may be treated in a logic circuit 31. The logic circuit providesan "advance" or "retraction" signal to a drive motor 32 for theelectrode 1 or the workpiece 2 in accordance with its collectiveprocessing results of discharge-characterizing signals received from thepulser 30. Obviously, the logic circuit 31 may be constituted by aplurality of preset counters and/or an up/down counter having suitableoutputs.

FIG. 8 shows a waveform diagram similar to that of FIG. 4 mentionedpreviously. The high-frequency component shown there superimposed on theDC component of discharge voltage and divided from the latter is shownin FIG. 9. It is shown that the high-frequency oscillation diminisheswith the lapse of time within a discharge and it has already beenpointed out that measurement of this component is meaningful only whenits dependency on the kind and change of machining fluid is taken intoconsideration. Such dependency may, however, be substantiallycompensated for if measurement is made of change in ΔV with time.

Thus, as shown in FIG. 9, the magnitudes of the high-frequencyoscillation at an instant to immediately following the commencement ofdischarge and at an instant tx in the midst of the discharge may bemeasured as ΔVo and ΔVx, respectively and a calculation made on thevalue (ΔVo-ΔVx)/(tx-to). If detection of the magnitude at the end ofdischarge is possible, (ΔVo-ΔVe)/(te-to) or (ΔVx-ΔVe)/(te-tx) may beused for "excellent", "good", "fine" or like classifications.

A circuit assembly shown in FIG. 10, designed to achieve this end,comprises a monitoring pulse supply 40 (as has been described) connectedacross a machining gap G between a tool electrode and a workpiece. Inthe FIGURE, a machining pulse supply is omitted. The circuit assemblyincludes a monitoring discharge detector 41, a filter 42 and anamplifier 43 as described in connection with FIG. 6, the amplifier 43providing an amplified DC signal representative of the magnitude(time-varying) of the high-frequency oscillatory component of thedischarge ΔV as has been described. On the other hand, the commencementof each monitoring discharge is detected by a detector 44 coupled to themachining gap G to trigger a timer 45 into activation, this timer upondeactivation at the instant to actuating a pulser 46. The pulser thusprovides a narrow checking pulse at the instant to. The timer 45 alsooperates a second timer 47 which upon deactivation at the instant txactivates a second pulser 48 which provides a narrow checking pulse atthis instant.

A latch circuit 49 is responsive both to the output of the amplifier 43and the checking pulser 47 for memorizing the high-frequency signal ΔVofrom the amplifier 43 at the instant to. A second latch circuit 50 isresponsive to the output of the amplifier 43 and to the checking pulser48 for memorizing the high-frequency output ΔVx of the amplifier 43 atthe instant tx. A clock oscillator 51 is provided to substitute for andcount latched signals. Thus, a counter 52 is provided to count thelatched signal ΔVo and a counter 53 counts the latched signal ΔVx. Acomparator 54 compares the outputs of counters 52 and 53 to provide atits output a difference ΔVo-ΔVx. The difference signal is substituted byclock pulses which are counted by a counter 55. A further counter 56substitutes the operating time tx-to of the timer 47 with clock pulses.The outputs of counters 55 and 56 are compared by a divider 57 whichprovides an output signal (ΔVo-ΔVx)/(tx-to), a change of thehigh-frequency component with time. This signal is in turn substitutedby clock pulses which are counted by a counter 58 which feeds into acomparator 59 having one or a plurality of threshold levels set thereinby a preset network 60 and with which the output of the counter 58 iscompared. The threshold levels are variably set in consideration of thekind or molecular weight of machining fluid used, parameters ofmachining pulses, i.e. pulse duration, pulse interval and peak current,the electrode polarity, materials of the electrode and the workpiece andother machining parameters.

The signal (ΔVo-ΔVx)/(tx-to) may typically be represented in volt permicrosecond. A gap condition tested by a monitoring pulse which resultsin this form of signal being 0.06 to 0.02 may be regarded as "excellent"or "good" and a gap condition with the signal being 0.1 to 0.07 may bedefined as "fine" or "not good". The corresponding signals are appliedto an indicator network 61 which may function to collectivelydiscriminate the input signals to provide control signals for themachining power supply, the electrode feed and retraction control systemthe machining fluid supply unit, etc.

Of course, with a monitoring pulse having a consequence of arcing orshort-circuiting, the absence of the high-frequency component gives anindication of such unacceptable gap characteristics. In accordance withthe detection method just described, however, more subtle and expandeddiscrimination is obtainable, i.e. characterization of two substantiallydistinct discharges both involving the presence of the high-frequencycomponent. For example, there may be a discharge in which thehigh-frequency component is existent at the instant to but it rapidlydiminishes so that it disappears in the midst of the discharge, i.e. atthe instant tx. Such pulse may itself not essentially be detrimental butrepresents an undesirable gap condition which invokes a succession ofmachining pulse with little stock removal or which leads to arcing.

Still another important characterization of gap discharges in accordancewith the invention lies in gas-phase discharges, metallic-phasedischarges and a combination thereof which has hitherto been commonlyregarded as "normal". As experimentation demonstrates, however, ametallic discharge is less efficient in stock removal 30% than agas-phase discharge. As experimentation indicates, a gas-phase dischargeoccurs with a gap voltage (discharge-sustaining) in the range of 15 to25 volts and a high-frequency component in the range of 5 to 20 volts. Ametallic-phase discharge occurs with a gap discharge of 10 to 15 voltsand a high-frequency component of 2 to 5 volts. The metallic dischargeappears to take place near the end of a discharge pulse or uponclarification a short-circuiting condition caused by machining chips. Anarc discharge is seen to be brought about under the condition in which adischarge point struck by a metallic-phase discharge is excessivelyheated and has a gap voltage of 6 to 10 volts and a high-frequencycomponent less than 1 volt. Thus, including an open-gap pulse, gapdischarges may be classified into about five categories.

In FIGS. 11A-11C there are shown different modes of characteristicdischarges which may occur with a single applied pulse. With a pulseshown at A, there develops a gas-phase discharge in the incipient stageof the pulse and, as gaseous vapors are mixed with metallic vapors andthe latter's concentration increases in the discharge column, it tendsto shift into a metallic-phase discharge. Typically, such a dischargepulse is optimum in machining performance. As shown at B, if a dischargepulse has a high content of metallic vapors from its incipient stage,this is a typical metallic-phase discharge. It is also possible, asshown at C, that a metallic discharge shifts into a mixed gas andmetallic discharge with the gaseous vapor concentration augmented.Machining pulses having characters shown at B and C are low in stockremoval performance. From FIG. 12, it is seen that the stock removal bya metallic discharge is 50 to 70% of that by a gas discharge and an arcdischarge permits removal of only several percent of the removal by thegas discharge. Naturally, normal discharges may be inferred to includegas-phase discharges to metallic-phase discharges while abnormaldischarges include arc discharges to short-circuiting.

In FIG. 14 there is shown a further circuit arrangement for detectingmonitoring pulses to discriminate between distinct discharges. As in theprevious embodiments, a monitoring pulse supply 40 provides across themachining gap G a monitoring pulse interposed into a succession ofmachining pulses supplied from a machining power source (not shown) andthe resulting gap voltage or current is sensed by a detector 70. Here,the detector 70 comprises three sensing networks 71, 72 and 73 connectedin parallel with one another across the machining gap G.

The sensing network 71 feeds a sensed DC signal into a threshold circuit74 which determines from the sensed discharge signal if the discharge isnormal or abnormal with reference to a threshold value set therein andprovides one or both of the corresponding signals which may be pulsed bya pulser or pulsers 75 whose output is applied to an up/down counter 76.The detector 72 senses a DC component of the discharge whereas thedetector 73 senses an AC (high-frequency) component of the discharge andtransforms it into a DC signal.

The senser 72 has a pair of threshold circuits 77 and 78 at its output.The threshold circuit 77 determines from the incoming signal if thedischarge is of gas-phase with reference to a threshold level settherein corresponding to the gas-phase gap voltage and, if determinedso, provides an output signal which operates a pulser 79 for issuing apulse. The output pulse of the pulser 79 is thus indicative of"gas-discharge" signal. The threshold circuit 78 determines from theincoming signal if the discharge is of metallic-phase with reference toa threshold level set therein corresponding to the metallic-phase gapvoltage and, if determined so, provides an output signal which operatesa pulser 80 for issuing a pulse. The output pulse of the pulser 80 isthus indicative of "metallic-discharge" signal.

The sensor 73 likewise has a pair of threshold circuits 81 and 82 at itsoutput. The threshold circuit 81 determines from the incominghigh-frequency signal if the discharge is of gas phase with reference toa threshold level set therein corresponding to the gas-phasehigh-frequency voltage component and, if determined so, provides anoutput signal which operates a pulser 83 for issuing a pulse. The outputpulse from the pulser 83 is thus indicative of "gas-phase" signal. Thethreshold circuit 82 determines from the incoming high-frequency signalif the discharge is of metallic phase with reference to a thresholdlevel set therein corresponding to the metallic-phase high-frequencyvoltage component and, if determined so, provides an output signal whichoperates a pulser 84 for issuing a pulse. The output pulse from thepulser 84 is thus indicative of "metallic-discharge" signal.

An AND gate 85 is provided responsive both to the outputs of pulser 79and 85 to provide an output signal when the "gas-discharge" signal isreceived from the two counters. Likewise a second AND gate 86 isprovided responsive both to the outputs of pulsers 80 and 85 to providean output signal when the "metallic-discharge" signal is received fromboth pulsers. The output of the first AND gate 85 has the first inputterminal of an OR gate 87 whose second input terminal is connected tothe output of the second AND gate 86 via a flip-flop circuit 88 andwhose output terminal is fed to the up/down counter 76.

The flip-flop 88 here serves as a divider for the "metallic-discharge"signal from the AND gate 86. The output of the OR gate thus provides,for registration at the up/down counter 76, a signal corresponding tothe number of gas-phase discharges plus one half the number ofmetallic-phase discharges. This takes into account the fact that, asdescribed hereinbefore, a metallic discharge is one half or so lessefficient in stock removal performance, than a gas-phase discharge. Thissignal may be used to level up the counter 76 one step every time itissues while the "abnormal discharge" signal received from the pulser 75may be used to level down the up/down counter 76. The output of theup/down counter 76 is provided with an indicator and/or a signalprocessing unit for controlling one or a plurality of machiningparameters as described hereinbefore.

We claim:
 1. A method of detecting gap conditions in an EDM process,which comprises the steps of:(a) applying across a machining gap asuccession of machining voltage pulses resulting in machining currentpulses having a pulse duration and peak current preselected to attain apredetermined machining consequence; (b) interposing into saidsuccession a plurality of monitoring voltage pulses each time-spaced andindependent from adjacent said machining voltage pulses, said monitoringvoltage pulses being capable of ionizing said gap and thereby effectingin the gap respective monitoring electrical discharges havingindividually a pulse duration and peak current preselected independentlyfrom said machining current pulses; (c) measuring a characteristic ofsaid monitoring electrical discharge, thereby classifying it into one ofat least two categories; and (d) ascertaining the gap condition inresponse to the result of step (c), each of said machining voltagepulses having its leading edge time-spaced from the trailing edge of themonitoring voltage pulse which precedes each said machining voltagepulse, each of said monitoring voltage pulses having its leading edgetime-spaced from the trailing edge of the machining voltage pulse whichprecedes each said machining voltage pulse.
 2. The method defined inclaim 1 wherein said monitoring electrical discharge has a pulseduration in the range between 5 and 20 microseconds and a peak currentin the range between 10 and 100 amperes.
 3. The method defined claim 2wherein the step (c) is carried out by measuring at least one of thedetectable gap variables of the gap voltage, the gap current, and thehigh-frequency component with said monitoring voltage pulse withreference to at least one corresponding threshold level.
 4. The methoddefined in claim 3 wherein the step (c) is carried out by measuring saidat least one of the detectable gap variables over each monitoringvoltage pulse.
 5. The method defined in claim 3 wherein the step (c) iscarried out by measuring said at least one of the detectable gapvariables during a portion of each monitoring voltage pulse.
 6. Themethod defined in claim 5 wherein said portion is the end of eachmonitoring voltage pulse.
 7. The method defined in claim 1 wherein saidcategories include a gas-phase discharge having a gap discharge voltagein the range between 15 and 25 volts and a high-frequency oscillatoryvoltage component in the range between 5 and 20 volts and ametallic-phase discharge having a gap discharge voltage in the rangebetween 10 and 15 volts and a high-frequency oscillatory component inthe range between 2 and 5 volts.
 8. The method according to claim 3wherein said step (c) is carried out by measuring a combination of thegap voltage and the high-frequency component superimposed thereon at afrequency of 1 to 30 MHz.
 9. The method defined in claim 3 wherein saidthreshold level is set as a function of the molecular weight ofmachining fluid used.
 10. An apparatus for detecting gap conditions inEDM processes, which comprises:(a) means for applying across a machininggap a succession of machining pulses; (b) means for presetting a pulseduration and peak current of said machining pulses; (c) means forinterposing at least one monitoring voltage pulse into said successionof machining pulses, with a leading edge of each monitoring voltagepulse time-spaced from the trailing edge of the machining voltage pulsewhich precedes each said monitoring voltage pulse, said monitoringvoltage pulse being capable of ionizing said gap and thereby effectingin the gap a monitoring electrical discharge; (d) means for presetting apulse duration and peak current of said monitoring voltage pulse; (e)means for measuring a characteristic of a gap current pulse resultingfrom said monitoring voltage pulse to classify it into one of at leasttwo categories; and (f) means responsive to the last-mentioned means forindicating the gap condition.
 11. The apparatus according to claim 10wherein said monitoring voltage pulse has a pulse duration set in therange between 5 and 20 microsecond and a peak current set in the rangebetween 10 and 100 amperes.
 12. The apparatus defined in claim 10wherein said means (a) comprises a first electronic switch connected inseries with a DC source and said gap and a pulser for intermittentlyenergizing said first switch, said means (c) including a secondelectronic switch connected in parallel with said first electronicswitch in series with said DC source and said gap and energizable bysaid pulser, said means (b) including first variable resistor meansconnected in series with said first switch, said DC source and said gap,and said means (d) including second variable resistor means connected inseries with said second switch, said DC source and said gap, said means(e) including:(e₁) a sensing network connected to said gap and triggeredby said monitoring voltage pulse at a time determined by the applicationthereof, said sensing network producing an output representing thevoltage across said gap upon its traversal by the monitoring voltagepulse, (e₂) a filter connected to said sensing network for abstractingfrom the output thereof a high-frequency component, (e₃) an amplifierconnected to said filter for transforming said high-frequency componentinto a DC signal representing the magnitude of the high-frequencycomponent, (e₄) a comparator having one input connected to saidamplifier and another input served by a reference signal source settablein accordance with the molecular weight of the machining fluid suppliedto said gap, said comparator having an output, and (e₅) a pulserconnected to said comparator for issuing pulses in response thereto,said means (f) including a counter responsive to said pulser.
 13. Theapparatus defined in claim 10 wherein said means (a) comprises a firstelectronic switch connected in series with a DC source and said gap anda pulser for intermittently energizing said first switch, said means (c)including a second electronic switch connected in parallel with saidfirst electronic switch in series with said DC source and said gap andenergizable by said pulser, said means (b) including first variableresistor means connected in series with said first switch, said DCsource and said gap, and said means (d) including second variableresistor means connected in series with said second switch, said DCsource and said gap, said means (e) including:(e₁) a sensing networkhaving an electrical output representing a discharge produced by amonitoring voltage pulse, (e₂) a threshold network connected to saidsensing network and responsive to said output for producing a signalupon said output passing a predetermined threshold level, (e₃) a pulserconnected to said threshold network for outputting pulses upon theproduction of a signal by said threshold network, and (e₄) a logiccircuit connected to the pulser of (e₃) including at least one counter,said means (f) including a motor operatively coupled to said gap forcontrolling the width thereof.
 14. The apparatus defined in claim 10wherein said means (a) comprises a first electronic switch connected inseries with a DC source and said gap and a pulser for intermittentlyenergizing said first switch, said means (c) including a secondelectronic switch connected in parallel with said first electronicswitch in series with said DC source and said gap and energizable bysaid pulser, said means (b) including first variable resistor meansconnected in series with said first switch, said DC source and said gap,and said means (d) including second variable resistor means connected inseries with said second switch, said DC source and said gap, said means(e) including:(e₁) a sensing network connected to said gap and triggeredby said monitoring voltage pulse at a time determined by the applicationthereof, said sensing network producing an output representing thevoltage across said gap upon its traversal by the monitoring pulsevoltage, (e₂) a filter connected to said sensing network for abstractingfrom the output thereof a high-frequency component, (e₃) an amplifierconnected to said filter for transforming said high-frequency componentinto a DC signal representing the magnitude of the high-frequencycomponent, (e₄) a detector responsive to the commencement of eachmonitoring pulse, (e₅) a checking pulse generator responsive to saiddetector to generate a narrow checking pulse, (e₆) a latch circuitresponsive to the output of said amplifier and to said checking pulsefor memorizing a high-frequency signal from said amplifier at twodistinct intervals controlled by said checking pulse, (e₇) respectivecounters for counting respective latched signals at the respectiveintervals and for producing a difference signal, (e₈) a divider providedwith a clockpulse source and responsive to said difference signal toform the quotient of the difference signal and a time signal, and (e₉)threshold circuit responsive to the quotient for outputting pulses uponthe passage of a signal representing said quotient through apredetermined threshold level, the latter process operating the means(f).
 15. The apparatus defined in claim 10 wherein said means (a)comprises a first electronic switch connected in series with a DC sourceand said gap and a pulser for intermittently energizing said firstswitch, said means (c) including a second electronic switch connected inparallel with said first electronic switch in series with said DC sourceand said gap and energizable by said pulser, said means (b) includingfirst variable resistor means connected in series with said firstswitch, said DC source and said gap, and said means (d) including secondvariable resistor means connected in series with said second switch,said DC source and said gap, said means (e) including:(e₁) a pluralityof sensing networks each connected across said gap in parallel with theothers, (e₂) respective threshold circuits responsive to said sensingnetworks and connected thereto for generating outputs upon detectedvalues exceeding and falling below respective threshold values, and (e₃)logic circuitry including cross-connected AND gates connected to theoutputs of said threshold circuits for producing a pulse output which isfed to said means (f).